Pancreatic Islets Express a Ca2+-independent Phospholipase A2 Enzyme That Contains a Repeated Structural Motif Homologous to the Integral Membrane Protein Binding Domain of Ankyrin

Abstract

Pancreatic islets express a Ca 2؉-independent phospholipase A 2 (CaI-PLA 2 ) activity that is sensitive to inhibition by a haloenol lactone suicide substrate that also attenuates glucose-induced hydrolysis of arachidonic acid from islet phospholipids and insulin secretion. A cDNA has been cloned from a rat islet cDNA library that encodes a protein with a deduced amino acid sequence of 751 residues that is homologous to a CaI-PLA 2 enzyme recently cloned from Chinese hamster ovary cells. Transient transfection of both COS-7 cells and Chinese hamster ovary cells with the cloned islet CaI-PLA 2 cDNA resulted in an increase in cellular CaI-PLA 2 activity, and this activity was susceptible to inhibition by haloenol lactone suicide substrate. The domain of the islet CaI-PLA 2 from amino acid residues 150 -414 is composed of eight stretches of a repeating sequence motif of approximately 33-amino acid residues in length that is highly homologous to domains of ankyrin that bind both tubulin and integral membrane proteins, including several proteins that regulate ionic fluxes across membranes. These findings complement previous pharmacologic observations that suggest that CaI-PLA 2 may participate in regulating transmembrane ion flux in glucose-stimulated ␤-cells.Glucose-induced insulin secretion from pancreatic islet ␤-cells requires that glucose be transported into the ␤-cell and metabolized (1). Signals derived from glucose metabolism result in inactivation of plasma membrane ATP-sensitive K ϩ channels (K ATP ), 1 membrane depolarization, activation of voltage-operated Ca 2ϩ channels, influx of Ca 2ϩ, and a rise in cytosolic [Ca 2ϩ ], which triggers insulin exocytosis (2). Stimulation of islets with glucose also induces hydrolysis of arachidonic acid from islet membrane phospholipids (3), and the resultant accumulation of nonesterified arachidonic acid (4) may facilitate Ca 2ϩ entry into ␤-cells (5) and amplify depolarization-induced insulin secretion (6).Hydrolysis of arachidonic acid from membrane phospholipids in glucose-stimulated islets appears to be mediated in part by a phospholipase A 2 (PLA 2 ) enzyme that is catalytically active in the absence of Ca 2ϩ and that is inactivated by a haloenol lactone suicide substrate (HELSS) (7-10). Treatment of islets with HELSS results in attenuation of the glucose-induced rise in ␤-cell cytosolic [Ca 2ϩ ] and in inhibition of insulin secretion (8 -10). The structure of islet Ca 2ϩ -independent phospholipase A 2 (CaI-PLA 2 ) is not known, but a CaI-PLA 2 enzyme has recently been cloned from CHO cells (11) and its sequence determined (GenBank accession number 115470). We report here the cloning, expression, and sequence analysis of a homologous enzyme from a rat pancreatic islet cDNA library (12). IN); rodent Chow 5001 was from Ralston Purina (St. Louis, MO); ampicillin and kanamycin were from Sigma; and D-glucose was from the National Bureau of Standards. Media included KRB (Krebs-Ringer bicarbonate buffer; 25 mM HEPES, pH 7.4, 115 mM NaCl, 24 mM NaHCO 3 , 5 mM KCl...

Modulation of the Pancreatic Islet β-Cell-delayed Rectifier Potassium Channel Kv2.1 by the Polyunsaturated Fatty Acid Arachidonate

“…These results suggest that muscarinic receptor occupancy increases ␤-cell Kv current inactivation by iPLA 2 ␤-regulated mechanism(s), and this results in a shorter time to peak Kv current response. Phospholipase A 2 Regulates Delayed Rectifier Current Activity-iPLA 2 ␤ is expressed by islet ␤-cells from rats (29), humans (47), and mice (48), and by several insulinoma cell lines (31,38,39). Upon activation, iPLA 2 ␤ hydrolyzes the sn-2 substituent from phospholipids substrates, such as phosphatidylcholine, to yield a free fatty acid and a lysophospholipid, such as LPC (44,54,55).…”

“…The phospholipid content of esterified AA is higher in pancreatic islets than in other tissues (23,25,26,29), and arachidonate is hydrolyzed from phospholipids upon stimulation of islets with secretagogues (13,14,27,31,36,44) and accumulates to micromolar concentrations (7,27,28) that are sufficient to modulate delayed rectifier Kv channel FIGURE 5. Effects of the muscarinic agonist carbachol on INS-1 cell-delayed rectifier currents.…”

“…Islet phospholipase A 2 (PLA 2 ) enzyme(s) activated by stimulation with glucose and/or by G-protein signaling mechanisms catalyze hydrolysis of arachidonic acid from ␤-cell membrane phospholipids and cause levels of free arachidonic acid to rise by up to 3-fold to the mid-micromolar range (7,13,14,27,28). Such concentrations of arachidonic acid amplify both the rise in ␤-cell [Ca 2ϩ ] and insulin secretion induced by glucose (7, 15-17, 23, 25, 27, 28).Pharmacologic inhibition or molecular biologic suppression of expression of the pancreatic islet ␤-cell Group VIA phospholipase A 2 (iPLA 2 ␤) enzyme (24,29,30) results in attenuation of insulin secretory responses induced by glucose and other secretagogues (18 -22, 24). The products of iPLA 2 ␤ action on its *…”

“…Pharmacologic inhibition or molecular biologic suppression of expression of the pancreatic islet ␤-cell Group VIA phospholipase A 2 (iPLA 2 ␤) enzyme (24,29,30) results in attenuation of insulin secretory responses induced by glucose and other secretagogues (18 -22, 24). The products of iPLA 2 ␤ action on its *…”

“…Pharmacologic inhibition or molecular biologic suppression of expression of the pancreatic islet ␤-cell Group VIA phospholipase A 2 (iPLA 2 ␤) enzyme (24,29,30) results in attenuation of insulin secretory responses induced by glucose and other secretagogues (18 -22, 24). The products of iPLA 2 ␤ action on its phospholipid substrates include a nonesterified fatty acid, such as AA, and a 2-lysphophosplipid, such as lysophosphatidylcholine (LPC), and ␤-cell levels of both products correlate with iPLA 2 ␤ expression level (21,22,31,32).…”